Method for making monolithic hollow waveguide

ABSTRACT

The present application discloses a method and apparatus for making a low cost and highly efficient hollow waveguide for transmitting electromagnetic radiation. The waveguide is made from a solid substrate monolithic hollow tube with a reflectivity enhancing dielectric film formed directly over the inner surface. The film can be formed by native chemical reactions, with the material of the monolithic, hollow tube. The present application also discloses a method of polishing and cleaning the inner surface of the hollow tube. One of the problems identified with the hollow metallic waveguide has been the poor surface finish on the inner wall of the hollow tube which results from the processes used to fabricate the metallic tube into final form. The coated reflectivity enhancing dielectric films more or less duplicate the surface roughness in the as formed tube thus seriously affecting the performance of the waveguide especially at the shorter wavelengths. The interior surface of the as received metal tubes can be significantly improved by chemically polishing and cleaning the tube and thereby improving the transmission characteristic of the wave guides for both infrared and visible wavelengths.

This application is a division of application Ser. No. 07/832,708, filedon Feb. 7, 1992, now U.S. Pat. No. 5,325,458.

FIELD OF THE INVENTION

This invention pertains to hollow waveguides for transmittingelectromagnetic radiation for a laser, particularly for use in medical,industrial and military applications.

BACKGROUND OF THE INVENTION

CO₂ lasers are now being extensively used in medical, industrial andmilitary applications, and a variety of optical fibers and hollowflexible waveguides have been proposed as the transmission medium forthese applications. However, only hollow waveguides have proven to beeconomically and commercially possible for transmitting a relativelyhigh flux of CO₂ laser energy.

Although dielectric-coated hollow waveguides for transmittingelectromagnetic waves have been studied since the 1950's, thefabrication method for dielectric-coated hollow flexible waveguides atinfrared CO₂ laser wavelengths was not reported until 1983. (M. Miyagi,et al., entitled "Fabrication of Germanium-coated Nickel HollowWaveguides for Infrared Transmission", Appl. Phys. Lett., Vol. 43, No.5, Sep. 1 1983.) Since then, various versions of that waveguide havebeen suggested and made by other groups.

Hollow waveguides are now used more often to transmit laser light inmedical and industrial applications. In particular, the transmission oflaser light at very different wavelengths (10.6 μm and 0.6328 μm) isideally suited for hollow waveguides. Rigid hollow waveguides made ofhollow alumina ceramic tubes encased in a stainless steel jacket havebeen used extensively to transmit a CO₂ laser light in rigid endoscopicapplications. (See U.S. Pat. No. 4,917,083, Apr. 17, 1990). Thesewaveguides, however are limited in both length and power handling andcan readily overheat or melt when the laser light is not properlylaunched into the waveguide. Also, ceramic tubes are not flexible due totheir inherent lack of ductility, precluding their use in flexiblewaveguides.

Some types of known hollow metallic waveguides are not flexible enoughand tend to overheat, limiting the maximum input power. Another plastictype of flexible hollow waveguide has a larger inside diameter and alower power transmission efficiency.

The following United States patents disclose hollow waveguides andlightpipes for lasers: U.S. Pat. No. 4,652,083 to Laakmann on Mar. 24,1987 for "Hollow Waveguide", U.S. Pat. No. 4,688,892 to Laakmann on Aug.25, 1987 for "Hollow Waveguide Having Disparate Dielectric overcoating",U.S. Pat. No. 4,688,893 to Laakmann on Aug. 25, 1987 for "HollowWaveguide Having Plural Layer Dielectric", U.S. Pat. No. 4,805,987 toLaakmann on Feb. 21, 1989 for "Hollow Waveguide Using A Low RefractiveIndex Inner Layer", U.S. Pat. No. 4,913,505 to Levy on Apr. 3, 1990,"Hollow Lightpipe and Method for Its Manufacture", U.S. Pat. No.4,930,863 to Croitoriu et al. on Jun. 5, 1990 for "Hollow FiberWaveguide And Method of Making Same", and U.S. Pat. No. 4,917,083 toHarrington et al. on Apr. 17, 1990 for "Delivery Arrangement for A LaserMedical System". These patents are hereby incorporated by reference.

SUMMARY OF THE INVENTION

None of the above referenced hollow waveguides are made by a method ofan enhancing dielectric film formed directly over the interior surfaceof a solid substrate monolithic hollow tube.

One of the difficulties in making such a directly coated hollowwaveguide is the limitation of the finish of the inner surface of themonolithic hollow tubes which results from the processes used tofabricate the tubes into their final form. The coated dielectric filmsmore or less duplicate the surface roughness in the formed tube, thusseriously affecting the performance of the waveguide transmissionespecially at shorter wavelengths. It is therefore desirable to have asuccessful method to grow a film directly over the interior surface of asolid monolithic tube to make commercial, hollow flexible and rigidwaveguides with a highly efficient power transmission for both infraredand visible lasers. The present invention provides such a low cost andhighly efficient manufacturing method.

It is also therefore desirable to have a successful method to polish andclean the inner surface of the as-received monolithic hollow tube andthereby significantly improve the interior optical smoothness of thetube before forming a reflectivity enhancing dielectric film on theinterior surface of the tube. The present invention provides such lowcost and highly efficient chemical polishing and cleaning method andapparatus.

The present invention provides a low cost and highly efficient method tomanufacture commercial, monolithic, hollow flexible and rigid waveguidesfor transmitting both infrared and visible laser radiations.

The present invention also provides an easy and simple method tomanufacture low cost, commercial, monolithic, hollow flexible and rigidwaveguides by a single layer dielectric film formed directly over theinner surface of solid substrate monolithic hollow tubes. The followingmetals are useful in making such tubes: silver, aluminum, gold, barium,chromium, copper, molybdenum, nickel, tin, tungsten, lead, zinc, ironand their alloys.

The present invention provides a method for growing a high reflectivitydielectric film directly over the inner surface of solid substratemonolithic hollow tubes with an optimum film thickness for both infraredand visible wavelengths.

The present invention further provides a method to obtain a highreflectivity dielectric film with an optimum film thickness for bothinfrared and visible laser wavelengths by a native chemical liquid, gasand/or vapor phase reaction between the material of a solid substratemonolithic hollow tube and the chemical reaction components, where thematerial of the tube becomes a component of the formed dielectric film.

The present invention provides a simple method for making low cost andhighly efficient hollow flexible and rigid waveguides for transmittingboth CO₂ laser and HeNe laser radiations, comprising a solid substratemonolithic hollow tube with a reflectivity enhancing dielectric filmformed directly over the interior surface of the tube by a nativechemical reaction. Preferred enhancing dielectric films provided by thepresent invention were chosen to be materials with a complex index ofrefractivity, where the real part n₁, the refraction index, is less thanabout 4.5 and the imaginary part k₁, the extinction coefficient, whichrelates to the absorption properties of the film, should be close tozero.

The present invention provides detail design results on optimum filmthickness of metal halide and metal oxide form inside the monolithichollow waveguide for both CO₂ laser and HeNe laser wavelengths. The filmthickness grown over the interior surface of a very small insidediameter monolithic tube can be controlled by a "weight gain" method.

According to the present invention, a dielectric film is formed over theinner surfaces of monolithic hollow tubes such that the coated tube iscapable of transmitting light, for example, of CO₂ and HeNe light. Thecost of the method described herein is less than the cost for the priorart processes, but such waveguides will be superior to competingtechnologies such as solid fibers and other types of hollow waveguides,particularly when a visible light is transmitted through the waveguide.

The present invention provides a means of improving the interior opticalsmoothness of monolithic hollow metal tube by polishing and cleaning theinterior to a mirror like finish. Thereafter, an appropriate dielectricthin film can then be formed directly over such interior of the metallictube for efficient transmission of electromagnetic radiations such asCO₂ laser and HeNe laser lights, for example.

The present invention also provides a highly efficient non-contactchemical polishing method to reduce the interior surface roughness ofthe as-received monolithic hollow tube by a factor of 100 times.

The present invention further provides a highly efficient non-contactchemical cleaning method to clean the interior surface of theas-received monolithic hollow tube without any surface damage after thenon contact chemical polishing reactions.

A preferred embodiment of the present invention provides a highlyefficient polishing and cleaning method to improve the interior surfacefinish of the monolithic hollow silver tube by a factor of over 100times.

In one embodiment of this invention, the monolithic hollow tubes aremade from metal and metal alloy, such as silver and sterling silver. Thepreferred metals and metal alloys hay, a high thermal conductivity,excellent ductility and excellent optical characteristics (reflectivityin the visible spectrum). It is also preferred that any such metal ormetal alloy be capable of reacting with a halogen to form a thin nativemetal halide film. More preferably, such films have a very uniformdielectric coating with good infrared properties and excellentadherence. Alternatively, metal oxide films can be created to provide auniform dielectric coating. Any conventional oxidizing agent effectiveto form the corresponding metal oxide may be used.

Some applications, such as medical endoscopic applications, require aCO₂ gas purge down the center of the waveguide. This gas purge preventsfluids and debris from being pushed up the waveguide by the in-situ gaspressure. However, CO₂ gas in a waveguide causes thermal lensing andother transmission losses which result in a heating of the waveguide.However, bends, and in particular tight bends, produce local heating.

The waveguides of the present invention have unmeasurable transmissionlosses when used straight and are capable of being bent to a very smallradii of curvature without significant losses in transmission.Furthermore, local heating due to tight bends, thermal lensing at highpowers or coating damage is dissipated by the high thermal conductivityof the waveguides of the present invention. Thus, the present inventionavoids dangerous local heating of the tube.

It is preferred in medical applications of the present invention thatthe dielectric coating be substantially insoluble in body fluids, salineor similar solutions.

The present invention provides a monolithic waveguide for transmittingelectromagnetic radiation comprising: a solid monolithic hollow tube anda thin, native, dielectric film up to about 20 microns in thicknessformed directly over an interior surface of the monolithic hollow tubewherein the film has a complex index of refractivity with a real part,n₁, less than about 4.5 and an imaginary part, k₁, substantially equalto zero. The dielectric film may be composed of a metal halide or metaloxide film and the tube may be composed of a material having a complexindex of refractivity with a real part n₂ greater than about 2.0 and animaginary part k₂ less than approximately 85. The waveguide may be foruse in a surgical laser.

Alternatively, the waveguide of the present invention may be used inindustrial applications where the light is conducted to the work throughsaid waveguide rather then by conventional means such as mirrors ormounting the laser directly on a X-Y stage that positions the beam.Since the waveguide of the present invention is flexible, a light dutyX-Y stage could be used to position the waveguide. In applications wherea laser is being added to existing equipment, the waveguide of thepresent invention provides an easy way to make this addition. The energydirected by the waveguide can be used as an industrial power deliverydevice for cutting, soldering, heat treatment, marking and otherprocessing steps. The waveguide of the present intention also provides ameans for welding, cutting, soldering and combinations thereof that areinaccessible using conventional approaches.

The present invention may further provide a method of manufacturing amonolithic hollow waveguide for transmitting electromagnetic radiation,comprising: polishing and cleaning an interior surface of a monolithichollow tube and forming dielectric thin films up to about 20 microns inthickness directly over the polished and cleaned interior surface of themonolithic hollow tube. The invention may further provide polishing andcleaning steps that may be one of the following: contact mechanicalcleaning and polishing, non-contact chemical cleaning and polishing or acombination of chemical and mechanical cleaning and polishing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. I is a diagrammatic representation of one form of the solidsubstrate monolithic hollow silver tube in accordance with the presentinvention;

FIG. 2 is a diagrammatic representation of a transverse cross sectionalview of the monolithic hollow tube of FIG. 1;

FIG. 3 is a diagrammatic representation of one form of adielectric-coated solid monolithic hollow silver waveguide constructedin accordance with the present invention;

FIG. 4 is a diagrammatic representation of a transverse cross sectionalview of the monolithic hollow silver waveguide of FIG. 3;

FIG. 5 is a graphic representation of a single bounce CO₂ laser power(10.6 μm) reflectivity as a function of thickness of the film formedover the inner surface of the tube in accordance with the presentinvention, where the film material has a complex index of refractivitywith a real part n₁ =2.2 and an imaginary part k₁ =0.0, and thesubstrate material of the monolithic hollow tube has a complex index ofrefractivity with a real part n₂ =14.4 and an imaginary part k₂ =56.9;

FIG. 6 is a graphic representation of a single bounce HeNe laser power(0.633 μm) reflectivity as a function of thickness of the film formedover the inner surface of the tube in accordance with the presentinvention, where the film material has a complex index of refractivitywith a real part n₁ =2.2 and an imaginary part k₁ =0.0, the substratematerial of the monolithic hollow tube has also a complex index ofrefractivity with a real part n₂ =14.4 and an imaginary part k₂ =56.9;

FIG. 7 is a graphic representation of a single bounce CO₂ laser power(10.6 μm) reflectivity as a function of thickness of the film formedover the inner surface of the tube in accordance with the presentinvention, where the film material has a complex index of refractivitywith a real part n₁ =1.35 and an imaginary part k₁ =0.0, and thesubstrate material of the monolithic hollow tube has a complex index ofrefractivity with a real part n₂ =26.0 and an imaginary part k₂ =67.3;

FIG. 8 is a graphic representation of a single bounce CO₂ laser power(10.6 μm) reflectivity as a function of thickness of the film formedover the inner surface of the tube in accordance with the presentinvention, where the film material has a complex index of refractivitywith a real part n₁ =1.95 and an imaginary part k₁ =0.0, and thesubstrate material of the monolithic hollow tube has a complex index ofrefractivity with a real part n₂ =14.4 and an imaginary part k₂ =56.9;

FIG. 9 is a graphic representation of a single bounce CO₂ laser power(10.6 μm) reflectivity as a function of thickness of the film formedover the inner surface of the tube in accordance with the presentinvention, where the film material has a complex index of refractivitywith a real part n₁ =4.0 and an imaginary part k₁ =0.0, and thesubstrate material of the monolithic hollow tube has a complex index ofrefractivity with a real part n₂ =8.7 and an imaginary part k₂ =58.2;

FIG. 10 is a graphic representation of the incident bounce angle (Φ) andthe number of internal bounces as a function of the radius of curvatureover a 50 cm long monolithic hollow bent silver waveguide with 1 mminside diameter (ID) for a laser beam propagating in the direction ofthe guide axis;

FIG. 11 is a graphic representation of the incident bounce angle (Φ) asa function of the radius of curvature over the 50 cm long monolithichollow bent silver waveguides of varying inside diameters (IDs) for alaser beam propagating in the direction of the guide axis;

FIG. 12 is a graphic representation of the internal bounce as a functionof the radius of curvature over the 50 cm long monolithic hollow bentwaveguides of varying inside diameters (IDs) for a laser beampropagating in the direction of the guide axis;

FIG. 13 is a graphic representation of a CO₂ laser power (10.6 μm)transmission efficiency as a function of the radius of curvature of amonolithic hollow bent silver waveguide with a 0.85 μm in thickness of asilver bromide film coated directly over the inner surface of 50 cm longmonolithic hollow silver tube having 1 mm inside diameter;

FIG. 14a is a diagrammatic representation of one form of the apparatusfor running the polishing solution through the silver tube;

FIG. 14b is a diagrammatic representation of one form of the apparatusfor ultrasonic cleaning in DI water;

FIG. 14c is a diagrammatic representation of one form of the apparatusfor running 50% ammonium hydroxide through the tube;

FIG. 14d is a diagrammatic representation of one form of the apparatusfor ultrasonic cleaning in 50% ammonium hydroxide;

FIG. 14e is a diagrammatic representation of one form of the alternativeto the apparatus of FIG. 1c & FIG. 1d for pumping 50% ammonium hydroxidethrough the tube;

FIG. 14f is a diagrammatic representation of one form of the apparatusfor pumping DI water through the tube;

FIG. 15 is experimental results of a CO₂ laser power (10.6 μm)transmission efficiency as a function of the radius of curvature of amonolithic hollow bent silver waveguide with about 0.8 m in thickness ofAgBr film coated directly over the inner surface of a 50 cm longmonolithic hollow silver tube having 1 mm inside diameter;

FIG. 16 is a diagrammatic representation of one form of the apparatus toform silver halide films by bromine and iodine liquid phase reactions inaccordance with the present invention;

FIG. 17 is a diagrammatic representation of one form of the apparatus toform silver halide films chlorine gas phase reactions in accordance withthe present invention;

FIG. 18 is a diagrammatic representation of one form of the apparatus toform silver halide films by iodine and bromine vapor phase reactions inaccordance with the present invention; and

FIG. 19 is a drawing of a rigid version of the waveguide of the presentinvention.

DETAILED DESCRIPTION

FIG. 1 is a diagrammatic representation of one form of the solidsubstrate monolithic hollow silver tube 10 made with a uniform material15 and FIG. 2 is a transverse cross-sectional view of FIG. 1. The crosssection may be of a variety of geometric configurations but ispreferably either circular or rectangular. In one embodiment of thepresent invention, a monolithic hollow tube 10 is used to transmitelectromagnetic radiation 20 at a certain wavelength provided theinterior surface 25 of the tube 10 is highly smooth. For instance, amonolithic hollow silver tube with a substantially perfectly smoothinner surface will be an excellent hollow waveguide for transmittingHeNe laser light (at about 0.6 μm) with as high as over about 96% powertransmission at a very low incident angle of Φ equal to about 75°.However, such a monolithic hollow silver tube will not transmit CO₂laser power (at about 10.6 μm) efficiently. The silver used for themonolithic hollow tube is about 99% pure and preferably commercial finegrade silver. The monolithic hollow tube of the present invention canalso be made of one or more of the following metals: aluminum, gold,barium, chromium, copper, molybdenum, nickel, tin, tungsten, lead, zinc,iron and their alloys.

As shown in FIG. 3, one form of the reflectivity enhancingdielectric-coated solid substrate monolithic hollow silver waveguide 30for transmitting electromagnetic radiation 35 at both visible andinfrared wavelengths made according to the present invention. Thismonolithic hollow waveguide 30 has only a single solid substratemonolithic hollow silver tube 10 with a native reflectivity enhancingdielectric film 40 formed directly over the inner surface 25 of thehollow silver tube 10, in which the material 15 of the tube 10 also actsas one of the chemical reaction components to form the reflectivityenhancing dielectric film 40. FIG.4 is a diagrammatic representation ofs transverse cross-sectional view of the monolithic hollow silverwaveguide of FIG. 3. Compared with existing hollow waveguides which allhave multiple layer tubes and multiple layer coating films, the presentinvention is the most efficient and economical preferred approach.

As also shown in FIG. 3 and FIG. 4, before growing any reflectivityenhancing dielectric film 40 over the inner surface 25 of the asreceived monolithic hollow silver tube 10, the inner surface 25 may bepolished and cleaned to improve the surface smoothness of the interiorwall 28 of the hollow silver tube 10. Frequently the as-receivedmonolithic hollow tube 10 as a result of the processes used to fabricatethe monolithic tube 10, has a the poor surface finish on the interiorwall 28. The formed reflectivity enhancing dielectric film 40substantially duplicates the surface roughness of the monolithic hollowsilver tube and may cause serious scattering losses. In particular, whena visible light transmits through a hollow waveguide 30, the scatteringlosses from the surface roughness may dominate any absorption lossesfrom the film. In one embodiment of the present invention, the interiorwall 28 of the as-received monolithic hollow silver tube 10 is polishedand cleaned by a non-contact chemical method, or a contact mechanicalmethod, such as tube brushes. This polishing and cleaning improves thelaser power transmission characteristics of the monolithic hollowwaveguide 30.

According to a preferred embodiment of the present invention, theinterior surface finish of the tube can be improved to desired level bythe non-contact chemical polishing and cleaning methods on the innersurfaces of the hollow silver tubes. A chemical polishing solution canbe developed solely for this purpose for different materials.

A useful chemical polishing solution for a silver tube was prepared bydissolving 65 gms of CrO₃ in 100 ml of DI (deionized) water and adding 5ml of pure HCl. This solution is a mixed acid solution.

Another preferred embodiment of the present invention uses a monolithichollow silver tube as the solid substrate and halide elements as thechemically reacting components. Since the tube drawing process may leadto a very poor inner surface on the tubes that might effect thewaveguides performance, it is preferred that the inner surface on whichthe film is grown is smooth and specular. Thus, polishing and cleaningof the silver tubes improves the inner surface finish. Thin filmdielectric coatings are later formed on these surfaces to enhance thetransmission of both the perpendicular (S) and parallel (P)polarizations of light bouncing down the interior surface. Theperformance of these waveguides is dependent on the inner surface finishof the monolithic hollow tube, the thickness of the thin film and themorphology of the film.

There are several halides which form on silver that have cubic crystalstructures, making their properties isotropic instead of birefringent.The isotropic properties are important because birefringent crystalgrowth in the film can contribute to excess scatter of light dependingon the crystal dimensions relative to the optical wavelength propagatingin a waveguide. The metal halide films in general are very adherent anddo not crack, flake or spall, even after repeated bending. Therefore,this approach is very well suited for making both flexible and rigidwaveguides. Other types of films such as metal oxide films could also bedeposited on the polished and cleaned inner surface of the monolithichollow silver tubes.

Low cost monolithic hollow silver waveguides according to the presentinvention are particularly useful for transmission of laser lights witha combination of very different wavelengths, such as a combination ofCO₂ laser (about 10.6 μm) and HeNe laser beams (about 0.6 μm) which arenow being extensively used in surgical, industrial and militaryapplications as well as other fields. To obtain an optimum waveguidefilm thickness for transmitting both about 10.6 μm and about 0.6 μmlaser beams, an average polarized laser power transmission is calculatedas a function of thickness of the film formed over the inner surface ofthe monolithic reflective metal such as silver tube at both about 10.6μm and about 0.6 μm, where the film material has a complex index ofrefractivity with a real part n₁ =about 2.2 and an imaginary part k₁=about 0, the substrate material of the monolithic hollow tube has alsoa complex index of refractivity with a real part n₂ =about 14.4 and animaginary part k₂ =about 56.9, and the results are as shown in FIG. 5and FIG. 6 respectively (these calculations model a silver halide filmformed directly over the inner surface of a monolithic hollow silversubstrate). The thickness of the film is related to its weight per unitof surface area. For example, in a silver tube with a 1 mm internaldiameter, the increase in mass of the tube due to the formation of asilver bromide (AgBr) film is related to the thickness of the silverbromide by the following formula: ##EQU1##

Low cost monolithic hollow waveguides with very high laser powertransmitting efficiency for both CO₂ and HeNe laser wavelengths are madeby controlling the thickness of the film formed over the interiorsurface of the monolithic hollow tube to within the "transmissionwindows" for the relevant wavelenghts. It is very difficult to measurethe film thickness inside the monolithic tube, especially for tubes witha very small inside diameter. Most available thin film measuringinstruments are designed only for a flat surface at a set of visiblewavelengths. According to another preferred embodiment of the presentinvention, the film thickness inside a very small inner diameter hollowtube is successfully measured by a "weight gain" method and, thereby,the thickness of the film formed over the interior surface of themonolithic hollow tube can be controlled accurately at the "transmissionwindows" for more than one laser as shown in FIG. 5 and FIG. 6.

The "weight gain" measuring method may include the following steps:measuring the dimensions and weight of the monolithic hollow tube;measuring the dimensions and weight of the monolithic hollow tube afterpolishing and cleaning processes; and measuring the weight of themonolithic hollow tube after forming the reflectivity enhancingdielectric over the inner surface of the polished and cleaned monolithichollow tube.

FIG. 7, FIG. 8 and FIG. 9 are graphic representations of a single bounceCO₂ laser power (about 10.6 μm) reflectivity as a function of thethickness of the film formed over the inner surface of the monolithichollow tube in accordance with the present invention for different filmmaterials over different substrate materials of tubes. The results showthat there are many "film thickness windows" within about 20 μmthickness of the film where a single bounce reflectivity of an averagepolarized CO₂ laser beam can be as high as about 97.5% at a very lowincident angle of Φ equal to about 70° measured from the normal, for afilm having a complex index of refractivity with a real part n₁ lessthan about 4.5 and an imaginary part k₁ equal to about zero, over asubstrate of the monolithic hollow tube having a complex index ofrefractivity with a real part n₂ greater than about 2.0 and an imaginarypart k₂ less than approximately 85. In other words, the low cost andhighly efficient monolithic hollow waveguide for transmittingelectromagnetic radiations can be made by a reflectivity enhancingdielectric film having a complex index of refractivity with a real partn₁ less than aboout 4.5 and an imaginary part k₁ equal to about zeroformed directly over the inner surface of different kind of solidsubstrate. Some monolithic hollow metal or metal alloy tubes which haveoptical properties similar to those of silver or silver alloys arelisted in Table 1.

FIG. 10 shows that the angle of incident ray (Φ) and the number ofinternal bounces as a function of the radius of curvature (R) over ahalf meter long monolithic hollow bent waveguide with 1 mm insidediameter (ID) for a laser beam traveling in the direction of the guideaxis. Considering a waveguide having a 30 cm radius of bend over thehalf meter long monolithic hollow waveguide, the propagating light willhave a total of about 15 internal bounces with an incident bounce angleof Φ equal to about 86.7°.

                  TABLE 1                                                         ______________________________________                                        THE OPTICAL PROPERTIES OF THE MONOLITHIC                                      HOLLOW TUBE MATERIALS AT 10.6 MICRON                                          Material  n      k         Reference                                          ______________________________________                                        Ag        14.4   56.9      10 (A. J. Moses)                                   Al        20.5   58.6      11 (K. Kudo, et al.)                               Au        17.1   55.9      11 (K. Kudo, et al.)                               Cr        11.8   25.9      12 (A. P. Lenham, et al.)                          Cu        14.1   64.3      11 (K. Kudo, et al.)                               Fe        9.63   28.5      12 (A. P. Lenham, et al.)                          Ni        9.08   34.8      12 (A. P. Lenham, et al.)                          Sn        17.4   43.5      11 (K. Kudo, et al.)                               W         10.7   31.0      12 (A. P. Lenham, et al.)                          Zn        15.8   48.7      11 (K. Kudo, et al.)                               ______________________________________                                    

The references listed in Table 1 are: A. J. Moses, "Optical MaterialProperties", IFI/Plenum Data Corporation, pp. 4-92, 1971; K. Kudo,"Tables of Fundamental Properties of Materials", Kyoritsu Shuppan, TokyoJapan, 1972; and A. P. Lenham, et al., "Optical Constants of TransitionMetals in the Infrared", J. Opt. Soc. Amer., Vol. 5, pp. 1137-1138,1966, and are hereby incorporated by reference.

Considering a laser light traveling in the direction of the guide axisinside a 50 cm long monolithic hollow bent waveguides of varying insidediameters (ID), the incident bounce angle (Φ) and the number of theinternal bounces as functions of the radius of curvature (R) are asshown in FIG. 11 and FIG. 12 respectively. As the waveguide ID isincreased, the number of internal bounces will decrease and the incidentbounce angle (Φ) will increase. The ID of the monolithic hollow tubemaking up the waveguide can be less than about 5 mm and is preferablyless than about 3 mm. A preferred embodiment of the present inventionuses an inside diameter equal to about 1 mm.

To demonstrate a theoretical model of laser power transmissionefficiency inside a monolithic hollow bent waveguide, we consider alaser beam propagating in the direction of the guide axis and thewaveguide having an optimum AgBr film thickness of about 0.85 μm coateddirectly over the inner surface of a half meter long monolithic hollowsilver tube with an about 1 mm inside diameter. The CO₂ laser powertransmission efficiency as a function of the radius of curvature of themonolithic hollow bent silver waveguide is shown in FIG. 13. Thecoupling losses, scattering losses and the Gaussian wave(electromagnetic field) propagation phenomenon are not modeled.Therefore, the experimental results on CO₂ laser power transmissionefficiency for a practical monolithic hollow bent silver waveguide (i.e.FIG. 15) is lower than what is shown in FIG.13.

The as received hollow silver tubes of required length are processed toobtain monolithic hollow metallic waveguides for CO₂ and HeNe lasertransmission. The inner surface of the tubes is chemically polished toimprove the inner surface finish. After an appropriate finish isachieved, a thin dielectric coating can be formed to enhance thetransmission.

One embodiment of the invention is shown in FIG. 19. The picturedwaveguide 50 is encased in a jacket 51 made of either plastic or metaland couples with a device that focuses the light (not shown) into theentrance 52 of the waveguide where it extends out of the jacket 51. Thejacket 51 in this embodiment has an outside diameter 54 of 3 mm and alength up to 500 mm (but not necessarily). It may also include a taperedtip 53. The waveguide of the present invention is preferably sealed inthe jacket. For example, the waveguide could be slid into the jacket andbonded in place. In a flexible embodiment of the present invention, athinner wall jacket provides the flexible properties desired, asdiscussed below. In particular, the wall of the jacket must bedetermined by the mechanical properties of the inner metal tubing,including its wall weight and the snap back properties of the plastictubing. The wall weight is related to the thickness of the jacket walls.A heavy wall plastic tube can force a permanently bent inner metal tubeto return to its original shape but may, therefore, lack sufficientflexibility. A compromise must be achieved to meet all the requirements.The plastics useful in such embodiments are Polysulfones, such as thosemarketed under the tradename UDEL™ Polyarysulfones, such as thosemarketed under the tradename RADEL™, Teflon and Polyimide.

The mechanical properties of a silver tube, for example, could be asfollows. It is preferred that the tubing used to form the hollow tube behard as drawn. It is further preferred that the tube is not annealed orotherwise treated to make it soft. It is also preferred that the tube becapable of maintaining itself as straight as possible to minimizetransmission losses. Soft tubes may accumulate many undulations fromhandling that might not be correctable later. In addition, it ispreferred that the tube bends over its entire length and not locally.Thus, hard tempered, springy tubes are preferred in the presentinvention.

A waveguide made of silver is very soft and ductile. The waveguide canbe drawn through a number of dies which will increase its hardnesssomewhat. However, it will still be relatively soft and when bent thewaveguide retains the bend and does not spring back to its originalshape. It is desirable to have a waveguide that is straight or bendssmoothly.

The use of a plastic jacket in the present invention has producedremarkable results. A jacket made of polysulfone or polyarysulfone canforce the waveguide to spring back to its original shape. The wallthicknesses of the plastic jacket and the silver tube are optimized sothat the plastic jacket is the dominant tube in the system. The jacketcan then straighten out any bends and can ensure that any bend is smoothand gradual. The waveguide will thus maintain its flexibility. Optimallythe monolithic hollow tube that makes up the waveguide will have a wallthickness of less than about 0.3 mm and the jacket for a flexiblewaveguide will have an optimum wall thickness of less than about 0.8 mm.A thicker, heavier wall plastic jacket can be used to make a rigidwaveguide, optimally its wall thickness may be about 1 mm.

The present invention uses the stiff properties of the plastic jacket tocontrol the type of bending that occurs in the waveguide. The presentinvention distributes the bends over the entire length of the waveguideand reverses any permanent bends. A plastic with a strong desire toretain its shape, such as RADEL™ Polysulfone or UDEL™ Polyarysulfone, isbest for this invention.

The following examples are based on the use of a silver tube as themonolithic hollow tube.

EXAMPLE 1

Polishing Solution Preparation And Testing

The chemical polishing solution was prepared by dissolving 65 gms ofCrO₃ in 100 ml of DI H₂ O and adding 5 ml HCl forming a mixed acidsolution. This solution was diluted 10 times before using. If moreconcentrated solution is desired, the stock solution can be diluted only2-3 times with DI water.

The diluted solution should be tested before usage. This was done bytaking 10 ml of freshly prepared solution in a small beaker. A piece wascut from the silver tube to be polished and immersed in the polishingsolution, and left in for 30 seconds. The piece was removed from thesolution and rinsed with DI water. A white, silver chloride film formedon the surface. This film was and should be removed completely from thesurface either by,

i) dropping the piece in a beaker having DI water and placing the beakerin an ultrasonic device; or

ii) preparing a solution of 50% NH₄ OH in DI water, dipping the pieceinto the solution and holding it while gently moving it in the solution.

The film should dissolve completely leaving a visibly polished surface.This turns the DI water white.

Any discoloration of the silver surface indicates that the solution hasto be readjusted. The hydrochloric acid concentration is crucial as thereactions are too slow in solutions that are too weak. However, anoverly strong concentrated hydrochloric acid solution forms black filmon the silver which cannot be removed completely and leaves the silversurface discolored, and it is preferred that such blackened tubes arenot used in the waveguide of the present invention.

EXAMPLE 2

Set Up The Polishing And Cleaning Apparatus

Before setting up the apparatus, all the glassware and fittings shouldbe cleaned with methanol and DI water. Preferred embodiments of theapparatus for cleaning and polishing are set up as shown in FIGS.14(a-f).

1) Apparatus for running the polishing solution through the silver tubeis shown in FIG. 14a. Referring to FIG. 14a, a separatory funnel 60 wasconnected through a valve 61 to one end of a tube fitting 62. Themonolithic hollow tube 63 was attached to the other end of the tubefitting 62. A beaker was placed under the silver tube 63 to catch thepolishing solution as it runs through the tube 63.

2) Apparatus for ultrasonic cleaning in DI water is shown in FIG. 14b.Referring to FIG. 14b, a glass beaker 66 that was large enough to holdthe waveguide 67 completely immersed in deionized water 65 was held inan ultrasonic cleaner 68 by a support 116, such as a ring stand.

3) Apparatus for running 50% ammonium hydroxide through the tube isshown in FIG. 14c. Referring to FIG. 14c, a separatory funnel 70 filledwith ammonium hydroxide 69 was connected through two valves 71, 117 to atube fitting 72. The waveguide 67 was connected to the tube fitting 72.A beaker 73 or other similar container was used to catch the ammoniumhydroxide that had run through the waveguide.

4) Apparatus for ultrasonic cleaning in 50% ammonium hydroxide is shownin FIG. 14d. Referring to FIG. 14d, a glass beaker 75 large enough tohold a waveguide 67 completely immersed in ammonium hydroxide was heldin an ultrasonic cleaner 76 by a support, such as a ring stand 118.

5) An alternative to the apparatus for running 50% ammonium hydroxidethrough the tube, as shown in FIG. 14c, is apparatus for pumping 50%ammonium hydroxide through the tube, as shown in FIG. 14e. Referring toFIG. 14e, a pump having an inlet and an outlet 77 was connected to apressure relief valve 78. The outlet of the pump was connected to acoupler 79 that held the waveguide 67 in a beaker filled with ammoniumhydroxide. The inlet to the pump was used as a suction 82 for drawingthe ammonium hydroxide back through the pump.

6) Apparatus for pumping DI water through the tube is shown in FIG. 14f.Referring to FIG. 14f, a pump 83 was connected to a pressure reliefvalve 84. The inlet of the pump was connected to the outlet of a DIwater filter 85. The inlet of the DI water filter 85 was connected tothe outlet of a prefilter 86. The inlet of the prefilter 86 extendedinto a beaker 88 filled with DI water. The DI water was drawn up throughthe filters 86, 85 to the pump 83. The outlet of pump 83 was connectedto coupler 87 that held the waveguide in the beaker 88 so that the DIwater could be pumped through.

EXAMPLE 3

Polishing And Cleaning Procedure for Silver Tube

1) The polishing mixed acid solution and the ammonium hydroxide solutionwere prepared. The apparatus as shown in FIGS. 14(a-f) were set up.

2) The as-received silver tube was rinsed with methanol and water.

3) Place the tube in an ultrasonic cleaning apparatus with sufficientNH₄ OH to completely cover the tube for 10 minutes to remove oxide orany other film that may have formed during fabrication. This steputilizes the apparatus shown in FIG. 14d.

4) Remove the tube and rinse it with DI water using, for example, theapparatus shown in FIG. 14f.

5) Fill the separatory funnel, 60 in FIG. 14a, with about 125 ml of thepolishing solution and then connect the tube to be polished to thefunnel.

6) Let the polishing solution run through the tube for a maximum of 5minutes. The polishing process is self limiting as the tube will clogwith reaction product, i.e., AgCl. The typical reaction time is about 5minutes for tubes with a 1 mm inside diameter.

7) Remove the tube and rinse with DI water. Place the tube in ultrasoniccleaning apparatus for DI water, as shown in FIG. 14b for about 30minutes to remove most of the reaction product. Make sure to pull thetube up a couple of times in the beginning to remove the slurry frominside and to avoid clogging the tube.

8) Remove the tube and connect it to the apparatus for running 50% NH₄OH through the tubes, as shown in FIG. 14c. Let the alkaline solutionrun for 5 minutes.

9) Remove the tube and place it in the ultrasonic cleaning apparatus in50% NH₄ OH, as shown in FIG. 14d. Leave the tube in for 15 minutes.

10) (Steps 10 and 11 are alternatives to steps 8 and 9) Place the tubein apparatus for pumping 50% NH₄ OH through the tubes, as shown in FIG.14e, for 10 minutes.

11) Remove the tube and rinse with DI water. Place it in apparatus forpumping DI water through the tube, as shown in FIG. 14f, for 10 minutes.

12) Remove the tube and let it dry for 30 minutes. During the last 5-10minutes, the tube can be heated with a heatgun while drying.

13) Steps 5-11 can be repeated if longer polishing times are required.Optimum time of polishing can be determined for a particular batch oftubes and then all tubes can be polished for that time. The polishingtime can vary from 30 seconds to 30 minutes.

The tube is now ready for further processing. Quality control at thispoint can be implemented by measuring HeNe laser transmission.

EXAMPLE 4

The Polishing and Cleaning Procedure for Silver Alloy Tubes

For polishing and cleaning silver-copper alloy tubes, the composition ofpolishing stock solutions are 80 ml H₂ SO₄ +20 ml HNO₃ (etchingsolution) and 55 gm CrO₃ +1 ml HCl+200 ml H₂ O (polishing solution).These are diluted 10 times before use. The solution of H₂ SO₄ : HNO₃=4:1 etched the copper from the silver surface and then a subsequentpolish using the silver polishing solution of 55 grm CrO₃ : 1 ml HCl:200ml H₂ O was done to improve the inner surface finish such silver-copperalloy tube. A similar polishing method can be used to improve theinterior surface smoothness for other silver alloy tubes and thesolutions for etching and polishing can used in alternating polishingcycles.

EXAMPLE 5

Procedure to Form Silver Halide Films By Bromine and Iodine Liquid PhaseReactions

The apparatus to form silver halide films by bromine and iodine liquidphase reactions are shown in FIG. 16. Referring to FIG. 16, a separatoryfunnel 118 was placed in a cooling bath funnel 90. The separatory funnelwas connected to a valve 119 that controls the flow of bromine or iodineinto the silver tube 95. Between the separatory funnel and the silvertube there was a valve for controlling the flow of methanol 91 andnitrogen 92. These valves 91, 92 were connected to an adaptor 93 andthen to a tube fitting 94 attached to one end of the silver tube 95. Theother end of the silver tube was connected to a universal adaptor 96which in turn was connected to a bent adaptor 97. The bent adaptor wasconnected to a dewer condenser 98 which emptied the run off from thesilver tube into a collection flask 99.

The chemical reaction procedure comprises the steps of:

1. All the glassware was rinsed with methanol and dried. All joints weresealed with parafilm or PTFE shrink wrapping to prevent contamination.

2. Ice can be and was added to a cooling bath or condenser funnel toreduce the temperature of liquid bromine or liquid iodine beforeentering the silver tube.

3. Bromine or iodine liquid was added to the cooling bath funnel. Ameasured amount of liquid bromine or iodine was allowed to run throughthe tube and collected at the end.

4. The bromine or iodine valve was closed and collection flask wasreplaced.

5. The methanol valve was opened and the apparatus was rinsed of anyresidual bromine or iodine in the tube.

6. The tube was removed and rinsed with methanol and dried by purging itwith nitrogen for approximately 15 minutes.

7. All traces of bromine or iodine were removed from the apparatusbefore processing the next tube.

EXAMPLE 6

Procedure to Form Silver Halide Films By Chlorine Gas Phase Reaction

The apparatus to form silver halide films by chlorine gas phasereactions are shown in FIG. 17. Referring to FIG. 17, a glass tube 100was used to support the silver tube 101. One end of the silver tube 101was connected to a gas washing bottle 120 filled with saturated aqueousalkali 140 for collecting and neutralizing the used gas. The other endof the silver tube 101 was connected to a tube fitting 102. A chlorinegas tank 104 was connected to the silver tube 101 through the tubefitting via a three way vacuum valve 103 used to switch between chlorineand nitrogen. A source for nitrogen (not shown) was also connected tothe three way valve 103.

The chemical reaction procedure comprises the steps of:

1. All the glassware was rinsed with methanol and dried.

2. The set up was purged with nitrogen.

3. The silver tube was connected to the fitting 102, as shown in FIG.17.

4. The silver tube was purged with nitrogen for about 15 minutes.

5. The three way vlave was operated to allow chlorine gas to go throughthe silver tube, as shown in FIG. 17.

6. After a specified reaction time of up to three hours, the three wayvalve was operated to purge the silver tube with nitrogen for about 15minutes.

EXAMPLE 7

Procedure to Form Silver Halide Films By Iodine and Bromine Vapor PhaseReactions

The apparatus to form silver halide films by iodine and bromine vaporphase reactions is shown in FIG. 18. Referring to FIG. 18, a glass tube106 was used to support the silver tube 107. One end of the silver tube107 was connected to a gas washing bottle 114 filled with saturatedaqueous alkali 150 for collecting and neutralizing the waste gas. Theother end of the silver tube 107 was connected to a tube fitting 108which is connected to one port of a first three way vacuum valve 109.Another port of the valve 109 was connected to a second three way vacuumvalve 110, one port of which was connected to an iodine bottle 111 andthe other port was connected to a bromine bottle 112. The last port ofthe first valve 109 was connected to a source for nitrogen (not shown).The iodine and bromine bottles 111, 112 were connected to a third threeway vacuum valve 113 whose third port was also connected to the sourcefor nitrogen.

The chemical reaction procedure comprises the steps of:

1. All the glassware was rinsed with methanol and dried.

2. The set up was purged with nitrogen.

3. The silver tube was connected to the fitting 108, as shown in FIG.18.

4. The silver tube was purged with nitrogen for about 15 minutes.

5. Bromine liquid and iodine crystals were placed in their respectivebottles.

6. Depending on required treatment, the valves for either iodine orbromine vapor were opened. The flow rate of nitrogen for treatment withiodine vapor is 2 scfh and for bromine vapor is 1 scfh.

7. After a specified reaction time of up to three hours, the valves wereclosed and the silver tube was then purged with nitrogen for about 15minutes.

What is claimed is:
 1. A chemical preparation method for a hollowwaveguide comprising contacting the interior surface of a monolithichollow tube with an aqueous acidic solution, wherein said acidicsolution is selected from the group consisting of chromic acid,hydrochloric acid and combinations thereof.
 2. The method of claim 1,wherein said aqueous acidic solution has a concentration of betweenabout saturated and about 1 percent saturated chromic acid and betweenabout 1% and about 10% hydrochloric acid.
 3. A chemical preparationmethod for a hollow waveguide comprising contacting the interior surfaceof a monolithic hollow tube with an about 30% to 80% aqueous solution ofammonium hydroxide.
 4. A method of preparing a monolithic silver hollowtube for use as a monolithic hollow waveguide comprising the stepsof,(a) rinsing said tube with a polar solution; (b) applying ultrasonicenergy to said tube; (c) contacting the interior surface of said tubewith a polishing solution; (d) contacting the interior surface of saidtube with an ammonium solution; and (e) drying said tube.
 5. A methodaccording to claim 4, which further comprises the steps of:(d1)inspecting said tube for oxides after having removed said tube from saidammonium solutions; and (d2) removing any oxides found in step (d1) . 6.A method of preparing a monolithic silver copper alloy hollow tube foruse as a monolithic hollow waveguide comprising the steps of(a)contacting the interior surface of said tube with a mixed acid solution;and (b) contacting said interior surface of said tube with an aqueousacidic solution comprising chromic and hydrochloric acids.
 7. The methodaccording to claim 6, wherein the mixed acid solution is prepared bydissolving about 65 gms of chromium trioxide in 100 ml of water andadding about 5 ml of hydrochloric acid.
 8. The method according to claim6, which further comprises repeating steps (a) and (b) at least threetimes.
 9. The method according to claim 6, in which said contact step iseffective to etch the interior surface of, said tube.